In a DVB-H system (DVB-H=digital video broadcasting handhelds), several multimedia services, in particular digital video signals, may be transmitted, in a transport stream, in time-division multiplexing via a transmission channel having a quasi-constant bit or data rate. If each video signal is assigned a fixed data or encoding rate, in accordance with an encoded information signal, a program provider, for example, will be forced to find a tradeoff between a transmission capacity that is sometimes expensive and a picture quality that may be achieved for critical scenes. Occasionally, a data or compression rate will not suffice in this context, and scenes that are rich in detail might suffer an impairment of quality. On the other hand, with a fixedly assigned encoding rate, it may also happen that the assigned encoding rate exceeds an encoding rate that may be used for a current scene, and that, therefore, the encoding rate, or cost, is wasted.
Depending on a current picture content, a video encoder may use different levels of data and/or encoding rates so as to ensure, for example, a high-quality television transmission. A sports broadcast, for example, typically uses a higher data rate—because its picture contents contain a lot of motion—than, e.g., a talk show or a news broadcast, which has rather static picture contents. Particularly high data rates are used for transmitting scenes that are rich in detail and contain a lot of motion.
Video encoding and/or compression methods are based, for example, on predictions, such as the so-called hybrid encoders, which perform, for a picture, a motion-compensated or picture-internal prediction with subsequent, e.g. entropy-based, compression of the remainder of the prediction. This means, similarities within a picture (intra) and/or among the pictures (inter) are exploited for making a prediction. Said predictions works with varying degrees of success, depending on the picture content. Accordingly, the residual signal will be larger or smaller, depending on the quality of the prediction. A larger residual signal uses a larger number of bits for encoding. Conversely, encoding of the motion compensation also involves, as side information, bits for encoding, so that a more complex prediction does not necessarily result in an improved compression rate. Overall, an ideal picture quality, or an ideal tradeoff between rate and quality, may be found for various data rates available and, therefore, for various compression rates. This relationship between the rate available and the picture quality achievable is dependent on the signal. Therefore, different compression rates or data rates are used for encoding for different scenes in order to achieve the same subjective picture quality.
The larger a number of programs or a number of program providers, the more unlikely it will be that all of the programs simultaneously use a very high data rate for being encoded. If several information signals, in particular videos, are transmitted in a transport stream via a channel having a constant overall data rate, said differences in the data rates may be exploited in assigning data rates to the individual services.
To this end, the individual data rates of a DVB-H network may be configured dynamically in accordance with a so-called statistic multiplex. This involves distributing the data rates such that a ratio between the encoding rate and the picture quality becomes optimal. This method is cooperative and involves that a sum of data rates of the individual services remains smaller than the available overall data rate. Instead of allotting a fixed data rate to each information signal, the statistical multiplexing analyzes contents of the picture material to be transmitted and assigns different individual data rates, depending on the prediction properties, to the plurality of information signals for shared transmission in a transport stream via the channel having the constant overall data rate. Instead of assigning a maximally used data rate to each video, it is thus possible to operate with a clearly reduced average data rate per video without reducing the picture quality perceived. Therefore, an overall impairment of quality may be reduced in this manner.
Reception of videos or information signals at a mobile terminal, such as a DVB-H receiver, should obviously not result in its battery being depleted within a very short time. For DVB-T systems (DVB-T=digital video broadcasting terrestrial), an entire data stream may be decoded before access may be made to any of the services contained within the data stream, such as TV programs, for example. With DVB-H, one uses the so-called time slicing technique, wherein only part, or a time slice, of the data stream is received which contains data of a service or program that has just been selected. With DVB-H, merging or multiplexing of different services is therefore performed in pure time-division multiplexing, wherein information signals of each service are periodically sent in compressed data packets or time slices. Thus, an individual service is not emitted continuously, but only from time to time and at a correspondingly high data rate, and sometimes it is not emitted at all. Time-division multiplexing of several services will then yield a continuous data stream with a quasi-constant data rate, as is shown in FIG. 7, for example.
FIG. 7 shows a continuous data stream 700 with a constant mean data rate BR. The data stream 700 represents an MPEG transport stream that is made up of MPEG-2-compatible (MPEG=moving picture experts group) elementary data streams organized into time slices 702. FIG. 7 reveals that program-specific information and meta data 704 (PSI/SI) are not subject to the time slicing method. In addition, neither a fixed, e.g. repetitive, assignment of the individual services to time slices 702, nor a fixed magnitude or duration of same is prescribed, even though in many DVB-H multiplexes such a fixed structure is used anyway. The duration of a time slice 702 associated with an information signal generally depends on the size of the current data packets of the respective service that are to be transmitted within said time slice. For example, if a video signal currently involves a comparatively high encoding rate, the time slice 702 that may be associated with the video signal will have a correspondingly long time duration.
Due to the variable time-slice structure depicted in FIG. 7, a receiver of the data stream 700 may have the precise position and configuration of the time slices 702 transmitted to it, so that the sequential data flow of the individual broadcasting services may be reconstructed therefrom. For DVB-H, the so-called delta-T method is used to this end. It includes transmitting, within each time slice 702, a relative waiting time delta-T informing the receiver when the next time slice of the same service is receivable. The system allows signaling waiting times within a range of a few milliseconds up to about 30 seconds (see FIG. 8).
Within the receiver, incoming time slices 702 are buffered and subsequently read out at a constant rate (at the average data or encoding rate of the respective service). The duration of the time slices 702 is typically within a range of several hundred milliseconds, whereas the switch-off time, in accordance with delta-T, of the receiver between the time slices may be many seconds (see above). Depending on the ratio of on/off-time, power savings of more than 90% as compared to DVB-T may result. For this purpose, time slicing presupposes a sufficient number of services or information signals in order to be as effective as possible.
In a DVB-H system, information signals or services are transmitted on the basis of the internet protocol (IP). This approach enables simple connection with other networks. The MPEG-2 transport stream 700 serves as a physical carrier. Embedding of IP data within the transport stream is effected by using an existing adaptation protocol, so-called multi-protocol encapsulation (MPE). To protect the transport stream 700 from interfering effects of a radio channel, DVB-H additionally comprises resorting to an error protection (MPE-FEC), which is applied at the level of the IP data stream before the IP data is encapsulated by means of MPE. By this mechanism, the receiving power is to be generally improved, in particular the reliability for mobile reception and with strong pulse-shaped interferences as may occur, for example, due to multipath propagation and resulting destructive interferences at the point of reception.
MPE-FEC is very similar to time slicing and MPE. These three techniques are directly tuned to one another, and together form the so-called DVB-H codec. IP data streams from the various sources are multiplexed as individual elementary streams by the time slicing method. The error protection MPE-FEC is calculated separately for each individual elementary stream and is added. This is followed by encapsulating the IP packets into the so-called sections of the multi-protocol encapsulation and, subsequently, by embedding into the transport stream.
With regard to temporal behavior, the disadvantage of the delta-T method is that the data or encoding rate of a DVB-H service can be changed only for the future time slices of the respective service. An algorithmic delay results, which corresponds to the distance of the time slices of the individual services. In the event of constant-rate services, this disadvantage is not relevant. In this case, the payload data of the services may be immediately encapsulated and combined into time slices.
This is different, however, for services that employ a variable data/encoding rate, such as in statistical multiplexing, for example. Due to the offset signaling, such services may be delayed in accordance with the repetition rate of the time slices assigned. To be able to indicate, in the time slices of a current transmission cycle, the respective relative waiting times delta-T up to the corresponding time slices of the subsequent transmission cycle, a time-slice structure, i.e. time-slice starting times and/or time-slice durations, of the subsequent transmission cycle ideally has already been recognized. After all, the data-rate requirements of a service may change from one time slice to its subsequent time slice, as a result of which a completely different time-slice structure may result for the subsequent transmission cycle as compared to the current transmission cycle.
In order to know future data-rate requirements or encoding-rate requirements, and, thus, the subsequent time-slice structure, for statistical multiplexing, the information signals may already be analyzed in advance. This may result in considerable latency periods. The time diagram of FIG. 9 shows an illustration of this circumstance.
FIG. 9 shows a temporal sequence of processing of an elementary stream 900 incoming for a service on the transmitter side. The data stream 900 is partitioned into portions N−1, N, N+1. The data of the portion N is to be transmitted within a time slice 902-N, and the data of the portion N+1 is to be transmitted within a subsequent time slice 902-(N+1). It may be seen from FIG. 9 that a data-rate analysis 904-(N+1) for the data-stream portion N+1 may have been completed by the time the time slice with the data of the respectively temporally preceding portion is sent, i.e. time slice 902-N. This is due to the fact that, as was already described above, the relative waiting time up to the subsequent time slice 902-(N+1) is integrated in the time slice 902-N. Thus, the data rate analysis for the portion N may already have been completed by the time-slice starting time TN−1 of the time slice 902-(N−1), the data-rate analysis of the portion N+1 may already have been completed by the time-slice starting time TN of the time slice 902-N, etc. This results in a relatively long latency period TL between the arrival of the data portions N, N+1 of the elementary stream 900 and the corresponding reproduction times or time-slice starting times TN or TN+1.
The above-described long latency period TL actually contradicts the goals of statistical multiplexing.